Ambient-activated switch for downhole operations

ABSTRACT

An apparatus includes a radiation source to output radiation after power is supplied. The apparatus also includes a power source to supply power. The apparatus includes an ambient-activated switch electrically coupled between the radiation source and the power source. The ambient-activated switch is to switch to an open position while a value for an ambient characteristic for the ambient-activated switch is at an ambient level for a surface of the Earth. The ambient-activated switch is to switch to a closed position while the value for the ambient characteristic for the ambient-activated switch is at a downhole ambient level, wherein the ambient-activated switch is to electrically couple the power source to the radiation source while in the closed position.

RELATED APPLICATIONS

This application is a nationalization under 35 U.S.C. 371 ofPCT/U.S.2007/020225, filed Sep. 18, 2007 and published as WO 2009/038554A1, on Mar. 26, 2009; which application and publication are incorporatedherein by reference and made a part hereof.

TECHNICAL FIELD

The application relates generally to downhole operations. In particular,the application relates to a ambient-activated switch for controllingcomponents downhole.

BACKGROUND

During drilling operations, determining characteristics (such as theporosity) of the subsurface formation enables the locating andextracting of hydrocarbons to be more efficient and more profitable. Onetechnique to determine these characteristics is to use radiation sources(such as pulsed neutron generators) downhole. The radiation sourcesoutput radiation into the subsurface formation. The resulting energyspectrum is monitored to determine the characteristics of the formation.The operation of these radiation sources in the presence of personnelcan adversely affect the health of such personnel. Accordingly, a numberof safety regulations for drilling operations require that such toolsnot be operational within a given distance of personnel.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention may be best understood by referring to thefollowing description and accompanying drawings which illustrate suchembodiments. In the drawings:

FIG. 1A illustrates a drilling well during wireline logging operations,according to some embodiments.

FIG. 1B illustrates a drilling well during Measurement While Drilling(MWD) operations, Logging While Drilling (LWD) operations or SurfaceData Logging (SDL) operations, according to some embodiments.

FIG. 2 illustrates part of a bottomhole assembly/tool body having ameasurement assembly that includes a radiation source, according to someembodiments.

FIG. 3 illustrates a configuration of an ambient-activated switch tocontrol power to a radiation source downhole, according to someembodiments.

FIG. 4 illustrates a configuration of an ambient-activated switch tocontrol power to a radiation source downhole, according to some otherembodiments.

FIG. 5 illustrates a configuration of an ambient-activated switch tocontrol power to a radiation source downhole, according to some otherembodiments.

FIG. 6 illustrates a configuration of an ambient-activated switch tocontrol power to a radiation source downhole, according to someembodiments.

FIG. 7 illustrates a configuration of an ambient-activated switch tocontrol power to a radiation source downhole, according to some otherembodiments.

DETAILED DESCRIPTION

Methods, apparatus and systems for ambient-activated switch(es) fordownhole operations are described. In the following description,numerous specific details are set forth. However, it is understood thatembodiments of the invention may be practiced without these specificdetails. In other instances, well-known circuits, structures andtechniques have not been shown in detail in order not to obscure theunderstanding of this description.

Some embodiments include a radiation source that is operational downholeto radiate the formation. Such radiation is used to determine variouscharacteristics of the formation. The radiation source can include apulsed neutron generator, an x-ray sources, etc. Some embodimentsinclude an ambient-activated switch coupled to supply power from a powersource to the radiation source. The switch may be activated based on oneor more ambient characteristics. For example, the switch may activatebased on temperature, pressure, light, vibration, etc. Accordingly, theswitch may be one or a combination of a thermal switch, a pressureswitch, an optical switch, a vibratory switch, etc. For example, thethermal switch may switch to a position to supply the power to theradiation source after a given temperature or temperature range isreached. The thermal switch may switch after a given downholetemperature is reached. Alternatively or in addition, the switch may bea pressure switch that switches to a position to supply the power to theradiation source after a given pressure or pressure range is reached.Alternatively or in addition, the switch may be an optical switch thatswitches to a position to supply power to the radiation source after theamount of light is below a given level. Alternatively or in addition,the switch may be a vibratory switch that switches to a position tosupply power to the radiation source after the amount of vibration isabove a given level. Accordingly, the radiation source may only becomeoperational downhole and not at the surface. Some embodiments may beused in different modes of conveyance including electric line,slickline, pipe conveyed logging or Logging While Drilling (LWD), robotconveyance, etc. Example system configurations are described below.

This description of the embodiments is divided into three sections. Thefirst section describes wellsite operating environments. The secondsection describes different configurations for controlling operations ofa radiation source using an ambient-activated switch. The third sectionprovides some general comments.

Wellsite Operating Environments

Wellsite operating environments, according to some embodiments, are nowdescribed.

FIG. 1A illustrates a drilling well during Measurement While Drilling(MWD) operations, Logging While Drilling (LWD) operations or SurfaceData Logging (SDL) operations, according to some embodiments. It can beseen how a system 164 may also form a portion of a drilling rig 102located at a surface 104 of a well 106. The drilling rig 102 may providesupport for a drill string 108. The drill string 108 may operate topenetrate a rotary table 110 for drilling a borehole 112 throughsubsurface formations 114. The drill string 108 may include a Kelly 116,drill pipe 118, and a bottom hole assembly 120, perhaps located at thelower portion of the drill pipe 118.

The bottom hole assembly 120 may include drill collars 122, a downholetool 124, and a drill bit 126. The drill bit 126 may operate to create aborehole 112 by penetrating the surface 104 and subsurface formations114. The downhole tool 124 may comprise any of a number of differenttypes of tools including MWD (measurement while drilling) tools, LWD(logging while drilling) tools, and others.

During drilling operations, the drill string 108 (perhaps including theKelly 116, the drill pipe 118, and the bottom hole assembly 120) may berotated by the rotary table 110. In addition to, or alternatively, thebottom hole assembly 120 may also be rotated by a motor (e.g., a mudmotor) that is located downhole. The drill collars 122 may be used toadd weight to the drill bit 126. The drill collars 122 also may stiffenthe bottom hole assembly 120 to allow the bottom hole assembly 120 totransfer the added weight to the drill bit 126, and in turn, assist thedrill bit 126 in penetrating the surface 104 and subsurface formations114.

During drilling operations, a mud pump 132 may pump drilling fluid(sometimes known by those of skill in the art as “drilling mud”) from amud pit 134 through a hose 136 into the drill pipe 118 and down to thedrill bit 126. The drilling fluid can flow out from the drill bit 126and be returned to the surface 104 through an annular area 140 betweenthe drill pipe 118 and the sides of the borehole 112. The drilling fluidmay then be returned to the mud pit 134, where such fluid is filtered.In some embodiments, the drilling fluid can be used to cool the drillbit 126, as well as to provide lubrication for the drill bit 126 duringdrilling operations. Additionally, the drilling fluid may be used toremove subsurface formation 114 cuttings created by operating the drillbit 126.

FIG. 1B illustrates a drilling well during wireline logging operations,according to some embodiments. A drilling platform 186 is equipped witha derrick 188 that supports a hoist 190. Drilling of oil and gas wellsis commonly carried out by a string of drill pipes connected together soas to form a drilling string that is lowered through a rotary table 110into a wellbore or borehole 112. Here it is assumed that the drillingstring has been temporarily removed from the borehole 112 to allow awireline logging tool body 170, such as a probe or sonde, to be loweredby wireline or logging cable 174 into the borehole 112. Typically, thetool body 170 is lowered to the bottom of the region of interest andsubsequently pulled upward at a substantially constant speed. During theupward trip, instruments included in the tool body 170 may be used toperform measurements on the subsurface formations 114 adjacent theborehole 112 as they pass by. The measurement data can be communicatedto a logging facility 192 for storage, processing, and analysis. Thelogging facility 192 may be provided with electronic equipment forvarious types of signal processing. Similar log data may be gathered andanalyzed during drilling operations (e.g., during Logging WhileDrilling, or LWD operations).

Configurations for Controlling Operations of a Radiation Source

FIG. 2 illustrates part of a bottomhole assembly/tool body having ameasurement assembly that includes a radiation source, according to someembodiments. FIG. 2 is described such that the measurement assembly ispart of the bottomhole assembly 120 (shown in FIG. 1B). However, someembodiments may be incorporated into the tool body 170 illustrated inFIG. 1A.

The bottomhole assembly 220 includes a radiation source 202 that iselectrically coupled to an ambient-activated switch 204. The switch 204may be activated based on one or more ambient characteristics. Forexample, the switch 204 may activate based on temperature, pressure,light, etc. Accordingly, the switch 204 may be one or a combination of athermal switch, a pressure switch, an optical switch, etc. The switch204 is electrically coupled to a power source 206. The radiation source202 may be a pulsed neutron generator, an x-ray generator, etc. Thepower source 206 may be representative of a battery, a fuel cell, aflow-driven generator, etc. In some embodiments, the power source 206 isnot part of the bottomhole assembly 220. For example the power source206 may be at the surface, while the radiation source 202 and the switch204 are in the bottomhole assembly 220.

In some embodiments, the radiation source, the switch and the powersource may be in one or more downhole tools of the bottomhole assembly.For example, the switch may be positioned in the downhole tool thatincludes the radiation source (e.g., the electronics of a pulsed neutrondownhole tool). The switch may also be in the power supply section ofthe radiation source. In some embodiments, the switch may be in astandalone sub located anywhere downhole. The switch may also beincorporated into a battery pack or between a battery pack and theinstrumentation section of a measuring device (such as a pulsed neutrontool).

The switch 204 may be different types of thermal switches that switchbetween an open and a closed position depending on the ambientcharacteristics. For example, the switch 204 may switch to a closedposition to allow for current flow after a temperature, pressure,measure of light is reached. In some embodiments, the switch 204 isconfigured to change to a closed position (to supply power to theradiation source 202) if predetermined functional ambient characteristicis higher than a level for the ambient characteristic at the surface ofthe Earth but lower than the level at the depth of needed function. Forexample, the switch 204 may be configured to allow for a current flowafter the ambient temperature is around or above a downhole temperature.Otherwise, the thermal switch 204 may remain in an open position. Theswitch 204 may remained in an open position while an ambient temperatureis around a surface temperature. Examples of thermal switches includeHoneywell 500 Series thermal switches, the Honeywell 250 Series thermalswitches, Honeywell 100 Series thermal switches, Honeywell 700 Seriesthermal switches, Honeywell 270 Series thermal switches, ControlProducts, Inc. (CPI®) SnapStat, Snap-Action thermal switches, ControlProducts, Inc. (CPI®) PlugStat Bimetallic Strip thermal switches,Control Products, Inc. (CPI®) Rod and Tube thermal switches, etc.Accordingly, the switch 204 may remain in a position not to supply powerwhile the ambient temperature is at or around a surface temperature. Theswitch 204 may switch to a position to supply power from the powersource 206 to the radiation source 202 after a downhole temperature isreached. While FIG. 2 illustrates one switch, more than one switch maybe used to control the power to the radiation source 202.

In some embodiments, the switch 204 is configured to be in an openposition and not supply the power to the radiation source 202 while theambient temperature is at or around a surface temperature. For example,the switch 204 may be in an open position in a range of less than about120 degrees Fahrenheit, a range of less than about 135 degreesFahrenheit, a range of less than about 150 degrees Fahrenheit, a rangeof about −50 to 150 degrees Fahrenheit, a range of less than about 150degrees Fahrenheit, a range of less than about 120 degrees Fahrenheit, arange of less than 200 degrees Fahrenheit, a range of −40 to 1500degrees Fahrenheit, etc. In some embodiments, the switch 204 isconfigured to be in a closed position and to supply power to theradiation source 202 while the ambient temperature is at or around adownhole temperature. For example, the switch 204 may be in a closedposition in a range of greater than about 120 degrees Fahrenheit, arange of greater than about 135 degrees Fahrenheit, a range of greaterthan about 150 degrees Fahrenheit, a range of about 150 to 500 degreesFahrenheit, a range of greater than about 200 degrees Fahrenheit, arange of greater than about 175 degrees Fahrenheit, a range of about 150to 375 degrees Fahrenheit, etc. In some embodiments, the thermalswitches change positions at a temperature from about 150 degreesFahrenheit to about 165 degrees Fahrenheit, from about 150 degreesFahrenheit to about 155 degrees Fahrenheit, from about 155 degreesFahrenheit to about 170 degrees Fahrenheit, 150, from about 165 degreesFahrenheit to about 175 degrees Fahrenheit, from about −70 degreesFahrenheit to about 500 degrees Fahrenheit, etc. In some embodiments,the switch 204 is configured to be in a closed position and to supplypower to the radiation source 202 while X degrees Fahrenheit above agiven temperature threshold. For example, X may be 30 degrees Fahrenheitand the given temperature threshold is 95 degrees Fahrenheit; X may be20 degrees Fahrenheit and the given temperature threshold is 100 degreesFahrenheit; X may be 50 degrees Fahrenheit and the given temperaturethreshold is 100 degrees Fahrenheit, etc.

In some embodiments, the downhole temperature may be lower than thesurface temperature. Accordingly, the switch 204 is configured to be ina closed position and supply the power to the radiation source 202 ifthe downhole temperature is below a given threshold. For example, theswitch 204 may be in an open position in a range of greater than about70 degrees Fahrenheit, a range of greater than about 50 degreesFahrenheit, a range of greater than about 90 degrees Fahrenheit, a rangeof about −50 to 150 degrees Fahrenheit, a range of less than about 150degrees Fahrenheit, a range of −40 to 1500 degrees Fahrenheit, etc. Theswitch 204 may be in a closed position (to supply power) in a range ofless than about 70 degrees Fahrenheit, a range of less than about 50degrees Fahrenheit, a range of greater than about 90 degrees Fahrenheit,a range of about 150 to 500 degrees Fahrenheit, a range of less thanabout 100 degrees Fahrenheit, a range of less than about 25 degreesFahrenheit, a range of about 150 to 375 degrees Fahrenheit, etc. In someembodiments, the switch 204 is configured to change to a closed position(to supply power to the radiation source 202) if predeterminedfunctional temperature is lower than the surface ambient temperature buthigher than the temperature at the depth of needed function.

In some embodiments, the switch 204 comprises a pressure switch. Thepressure switch may remain in a position not to supply power while theambient pressure is at or around a pressure at the Earth's surface. Thepressure switch may switch to a position to supply power from the powersource 206 to the radiation source 202 after a downhole pressure isreached. For example, the pressure switch may be in an open position ina range of less than about 101 kiloPascals (kPa), a range of less thanabout 100 kPa, a range of about 90 to 110 kPa, a range of less thanabout 120 kPa, a range of less than about 150 kPa, etc. The pressureswitch may be in a closed position in a range of greater than about 110kPa, a range of greater than about 120 kPa, a range of greater thanabout 130 kPa, a range of greater than about 140 kPa, a range of greaterthan about 150 kPa, a range of greater than about 200 kPa, etc.

In some embodiments, the switch 204 comprises an optical switch. Theoptical switch may remain in a position not to supply power while theambient light is at or around a level of light at the Earth's surface.The optical switch may switch to a position to supply power from thepower source 206 to the radiation source 202 after the ambient lightlevel drops below a certain threshold. For example, the optical switchmay be in an open position in a range of greater than 0 candela persquare meters (cd/m²), greater than 1.0 cd/m², greater than 5 cd/m²,etc. The optical switch may be in a closed position in a range of lessthan about 0 cd/m², less than about 0.5 cd/m², less than about −0.5cd/m², less than about −1.0 cd/m², less than about −10.0 cd/m², etc.

In some embodiments, the switch 204 comprises a vibratory switch. Forexample, the vibratory switch may comprise any type of vibratory sensor(such as a gyroscope, an accelerometer, etc.). The vibratory switch mayswitch to a position to supply power from the power source 206 to theradiation source 202 after the vibration level exceeds a certainthreshold. For example, the vibratory switch may be in an open positionin a range of vibration of more than about 0.1 Gs, more than about 1 Gs,more than about 5 Gs, more than about 10 Gs, more than about 50 Gs, morethan about 100 Gs, etc.

The switch 204 may be positioned in any location in an electricalconfiguration to regulate the supply of power from the power source 206to the radiation source 202. FIGS. 3-7 illustrate exampleconfigurations.

FIG. 3 illustrates a configuration of an ambient-activated switch tocontrol power to a radiation source downhole, according to someembodiments. A circuit 300 comprises a power source 302, a switch 304and a radiation source 306. The power source 302 is coupled in serieswith the switch 304, which is in series with the radiation source 306.While a value for an ambient characteristic (such as temperature,pressure, light, vibration, etc.) of the switch 304 is at an ambientlevel for the surface of the Earth, the switch 304 is configured to bein an open position (see ranges described above). Accordingly, currentdoes not flow from the power source 302 to the radiation source 306.After the value for the ambient characteristic of the switch 304 reachesa given downhole ambient level, the switch 304 changes to a closedposition (see ranges described above). Therefore, current from the powersource 302 is supplied to the radiation source 306. Thus, the radiationsource 306 does not become operational until a value for a given ambientcharacteristic has reached a downhole ambient level.

FIG. 4 illustrates a configuration of an ambient-activated switch tocontrol power to a radiation source downhole, according to some otherembodiments. A circuit 400 comprises a power source 402, a switch 404and a radiation source 406. The circuit 400 is configured in a shuntconfiguration. The switch 404 is coupled in parallel between the powersource 402 and the radiation source 406. While a value for an ambientcharacteristic for the switch 404 is at an ambient level for the surfaceof the Earth, the switch 404 is closed thereby providing a shunt suchthat current does not flow to the radiation source 406. After the valuefor the ambient characteristic of the switch 404 reaches a givendownhole ambient level, the switch 404 switches to an open position.Therefore, current from the power source 402 is supplied to theradiation source 406. Thus, the radiation source 406 does not becomeoperational until a vale for a given ambient characteristic has reacheda downhole ambient level.

FIG. 5 illustrates a configuration of an ambient-activated switch tocontrol power to a radiation source downhole, according to some otherembodiments. A circuit 500 comprises a power source 502, a switch 504, aswitch 505 and a radiation source 506. The power source 502 is coupledin series with the switch 505, which is in series with the radiationsource 506. The circuit 500 is also configured in a shunt configuration.The switch 504 is coupled in parallel between the power source 502 andthe radiation source 506.

While a value for an ambient characteristic of the switch 505 is at anambient level for the surface of the Earth, the switch 505 is configuredto be in an open position. Accordingly, current does not flow from thepower source 502 to the radiation source 506. After the value for theambient characteristic of the switch 505 reaches a given downholeambient level, the switch 505 changes to a closed position. While avalue for the ambient characteristic of the switch 504 is at an ambientlevel for the surface of the Earth, the switch 504 is closed therebyproviding a shunt such that current does not flow to the radiationsource 506. After the value for the ambient characteristic of the switch505 reaches a given downhole ambient level, the switch 504 switches toan open position. Therefore, current from the power source 502 issupplied to the radiation source 506 until the switch 505 is in a closedposition and the switch 504 is in an open position. Thus, multipleswitches provide a redundancy to ensure that the radiation source is notsupplied power until a downhole ambient levels are reached.

FIG. 6 illustrates a configuration of an ambient-activated switch tocontrol power to a radiation source downhole, according to someembodiments. In particular, a circuit 600 comprises multiple switchescoupled in series between the power source and the radiation source. Thecircuit 600 comprises a power source 602, switches 604A-604N and aradiation source 606. The power source 602 is coupled in series with theswitches 604A-604N, which are in series with the radiation source 606.While a value for an ambient characteristic of the switches 604 is at anambient level for the surface of the Earth, the switches 604 areconfigured to be in an open position. Accordingly, current does not flowfrom the power source 602 to the radiation source 606. After the valuefor the ambient characteristic reaches a given downhole ambient level,the switches 604 change to a closed position. Therefore, current fromthe power source 602 is supplied to the radiation source 606. Thus, theradiation source 606 does not become operational until a given downholeambient level is reached. As described, any number and type of switches604 may be connected in series to ensure that current is not supplied tothe radiation source 608 until a given downhole ambient level isreached.

FIG. 7 illustrates a configuration of an ambient-activated switch tocontrol power to a radiation source downhole, according to some otherembodiments. In particular, a circuit 700 comprises multiple switchescoupled in a shunt configuration between the power source and theradiation source. The circuit 700 comprises a power source 702, switches704A-704N and a radiation source 706. The circuit 700 is configured in ashunt configuration. The switches 704 are coupled in parallel betweenthe power source 702 and the radiation source 706. While a value for anambient characteristic is at an ambient level for the surface of theEarth, the switches 704 are closed thereby providing a shunt such thatcurrent does not flow to the radiation source 706. After the value forthe ambient characteristic reaches a given downhole ambient level, theswitches 704 switches to an open position. Therefore, current from thepower source 702 is supplied to the radiation source 706. Thus, theradiation source 706 does not become operational until a given downholeambient level is reached. As described, any number and type of switches704 may be connected in parallel to ensure that current is not suppliedto the radiation source 708 until a given downhole ambient level isreached.

FIGS. 3-7 illustrates a number of example configurations. One to anynumber of ambient-activated switches may be positioned between the powersource and the radiation source to ensure that current is only suppliedto the radiation source after a given ambient characteristic is reacheda given threshold. An additional example may include the combination ofthe configurations shown in FIGS. 6-7. Specifically, multiple switchesmay be coupled in series and multiple switches may be coupled inparallel between the power source and the radiation source.

General

In the description, numerous specific details such as logicimplementations, opcodes, means to specify operands, resourcepartitioning/sharing/duplication implementations, types andinterrelationships of system components, and logicpartitioning/integration choices are set forth in order to provide amore thorough understanding of the present invention. It will beappreciated, however, by one skilled in the art that embodiments of theinvention may be practiced without such specific details. In otherinstances, control structures, gate level circuits and full softwareinstruction sequences have not been shown in detail in order not toobscure the embodiments of the invention. Those of ordinary skill in theart, with the included descriptions will be able to implementappropriate functionality without undue experimentation.

References in the specification to “one embodiment”, “an embodiment”,“an example embodiment”, etc., indicate that the embodiment describedmay include a particular feature, structure, or characteristic, butevery embodiment may not necessarily include the particular feature,structure, or characteristic. Moreover, such phrases are not necessarilyreferring to the same embodiment. Further, when a particular feature,structure, or characteristic is described in connection with anembodiment, it is submitted that it is within the knowledge of oneskilled in the art to affect such feature, structure, or characteristicin connection with other embodiments whether or not explicitlydescribed.

In view of the wide variety of permutations to the embodiments describedherein, this detailed description is intended to be illustrative only,and should not be taken as limiting the scope of the invention. What isclaimed as the invention, therefore, is all such modifications as maycome within the scope and spirit of the following claims and equivalentsthereto. Therefore, the specification and drawings are to be regarded inan illustrative rather than a restrictive sense.

What is claimed is:
 1. An apparatus comprising: a neutron radiationsource to output neutron radiation after power is supplied; a powersource to supply the power; and an ambient-activated optical switchelectrically coupled between the neutron radiation source and the powersource, the ambient-activated optical switch being configured to switchto a decoupling condition while ambient light at the ambient-activatedoptical switch is at an ambient level for a surface of the Earth, theneutron radiation source being electrically decoupled from the powersource while the ambient-activated optical switch is in the decouplingcondition, the ambient-activated optical switch being configured toswitch to a coupling condition while the ambient light at theambient-activated optical switch is at or below a threshold level,wherein the ambient-activated optical switch is configured toelectrically couple the power source to the neutron radiation sourcewhile in the coupling condition.
 2. The apparatus of claim 1, whereinthe neutron radiation source is a pulsed neutron tool.
 3. The apparatusof claim 1, wherein the ambient-activated optical switch is electricallycoupled in series between the neutron radiation source and the powersource, switching of the ambient-activated optical switch to thecoupling condition comprising switching thereof to a closed position. 4.The apparatus of claim 1, wherein the ambient-activated optical switchis electrically coupled in a shunt configuration between the neutronradiation source and the power source, switching of theambient-activated optical switch to the coupling position comprisingswitching thereof to an open position.
 5. The apparatus of claim 1,wherein the power source comprises a downhole power supply.
 6. Theapparatus of claim 1, wherein the neutron radiation source is to outputneutron radiation during a Measurement While Drilling Operation.
 7. Theapparatus of claim 1, further comprising a different ambient-activatedswitch electrically coupled between the neutron radiation source and thepower source, the different ambient-activated switch to switch to adecoupling condition while a value for an ambient characteristic for thedifferent ambient-activated switch is at an ambient level at a surfaceof the Earth, the different ambient-activated switch is to switch to acoupling condition while the value for the ambient characteristic forthe different ambient-activated switch is at a downhole ambient level,wherein the different ambient-activated switch is to electrically couplethe power source to the neutron radiation source while in the couplingcondition.
 8. The apparatus of claim 7, wherein the ambient-activatedswitch and the different ambient-activated switch are electricallycoupled in a shunt configuration between the neutron radiation sourceand the power source.
 9. The apparatus of claim 7, wherein the differentambient-activated switch comprises a pressure switch that is to switchto the decoupling condition while the ambient pressure for the pressureswitch is at pressure for the surface of the Earth, the pressure switchto switch to the coupling condition while the ambient pressure for thepressure switch is at a downhole level.
 10. The apparatus of claim 7,wherein the different ambient-activated switch comprises a switchselected from a group consisting of a thermal switch and a vibratoryswitch.
 11. The apparatus of claim 1, wherein the threshold level isabout 0 candela per square meter (cd/m²).
 12. The apparatus of claim 1,wherein the threshold level is in the range of 0-1 cd/m².
 13. Theapparatus of claim 1, wherein the threshold level is in the range of 1-5cd/m².
 14. A system comprising: a drill string having a drill bit, thedrill string to extend through at least part of a well bore, wherein thedrill string comprises: a tool to emit neutrons for formationevaluation; a power source to supply power; and a switch electricallycoupled between the tool and the power source, the switch beingoptically sensitive and being configured to switch to a condition inwhich it does not electrically couple the power source to the tool if avalue of an ambient light level at the switch is at an ambient level ata surface of the Earth, the switch being configured to switch to acondition in which it electrically couples the power source to the toolif the value of the ambient light level of the switch is at or below athreshold level.
 15. The system of claim 14, wherein the switch iselectrically coupled in series between the tool and the power source.16. The system of claim 14, wherein the power source comprises abattery.
 17. The system of claim 14, wherein the switch furthercomprises a thermal switch that is to switch to not electrically couplethe power source to the tool if an ambient temperature for the switch isat a temperature for the surface of the Earth, the thermal switch toswitch to a closed position while the ambient temperature for thethermal switch is at a downhole temperature.
 18. The system of claim 17,wherein the temperature for the surface of the Earth is in a range ofless than about 150 degrees Fahrenheit.
 19. The system of claim 17,wherein the downhole temperature is in a range of greater than about 200degrees Fahrenheit.
 20. The system of claim 14, wherein the switchfurther comprises a vibratory switch that is to switch to an openposition while the ambient vibration for the vibratory switch is at avibratory level for the surface of the Earth, the vibratory switch toswitch to a closed position while the ambient vibration for thevibratory switch is at a downhole level.
 21. The system of claim 14,wherein the switch, in addition to being optically sensitive, furthercomprises two or more switches selected from a group consisting of athermal switch, a pressure switch, and a vibratory switch.
 22. Thesystem of claim 14, wherein the threshold level is in the range of 0-5cd/m².
 23. A method comprising: electrically coupling a radiation sourceto an ambient-activated switch; electrically coupling theambient-activated switch to a power source, wherein theambient-activated switch is in series or in a shunt configurationbetween the radiation source and the power source, the ambient-activatedswitch being configured to switch to a decoupling condition while anambient light value of the ambient-activated switch is about an ambientlevel for a surface of the Earth, the radiation source beingelectrically disconnected from the power source while theambient-activated switch is in the decoupling condition, theambient-activated switch further being configured to switch to acoupling condition while the ambient light value of theambient-activated switch is at or below a threshold level, theambient-activated switch to electrically connect the power source to theradiation source while in the coupling condition; lowering a tool thatcomprises the radiation source and the ambient-activated switch down theborehole, the power source not being electrically coupled to the neutronradiation source at or adjacent the surface of the Earth; and uponreaching a depth at which an ambient light value sensed by theambient-activated switch is at or below the threshold level, switchingthe ambient-activated switch to the coupling condition, to power theradiation source from the power source.
 24. The method of claim 23,wherein the ambient-activated switch further comprises a thermal switchthat is to switch to not electrically couple the power source to theradiation source if an ambient temperature for the switch is at atemperature for the surface of the Earth, the thermal switch to switchto electrically couple the power source to the radiation source whilethe ambient temperature for the thermal switch is at a downholetemperature.
 25. The method of claim 24, wherein the temperature for thesurface of the Earth is in a range of less than about 120 degreesFahrenheit.
 26. The method of claim 24, wherein the downhole temperatureis in a range of about 150 to 375 degrees Fahrenheit.
 27. The method ofclaim 23, wherein the ambient-activated switch further comprises apressure switch that is to switch to a decoupling condition whileambient pressure for the pressure switch is at pressure for the surfaceof the Earth, the pressure switch to switch to a coupling conditionwhile the ambient pressure for the pressure switch is at a downholelevel.
 28. The method of claim 23, wherein the ambient-activated switchfurther comprises a vibratory switch.
 29. The method of claim 23,wherein the threshold level is in the range of 0-5 cd/m².